The invention relates generally to a system and method for controlling ignition in a homogeneous charge compression ignition (HCCI) engine by injecting a pilot fuel in conjunction with a main fuel. The system and method of this invention may be used to advantage in diesel fuel engines, and especially locomotive engines, as well as any other reciprocating engine.
Diesel engines efficiently convert the latent heat of hydrocarbon fuel into useful mechanical power. In the operation of conventional diesel engines, a metered amount of fuel is injected into each cylinder of the engine at recurrent intervals synchronized with rotation of the engine crankshaft to coincide with the air-compression stroke of a reciprocating piston. The compression of the air charge greatly increases its temperature. The fuel is sprayed into the cylinder near the top of the piston stroke where it quickly ignites in the high temperature air. The resulting combustion or firing of fuel in the cylinder forces the piston to move in the opposite direction, thereby applying torque to the engine camshaft.
Conventional diesel engine fuel is a relatively low grade, refined petroleum known generally as diesel fuel oil that has desirable ignition and heat release characteristics. Diesel fuel oil has acceptably low levels of corrosive, abrasive and other noxious matter, and it is in ample supply at the present time.
Diesel engines typically burn fuel in a diffusion combustion mode. In that mode, the fuel burns as it comes into the chamber before it is well mixed with air. Since only pure air is compressed in the compression stroke of the engine, a high compression ratio can be used to obtain high cycle efficiency. However, the local combustion air to fuel ratio cannot be controlled. Much of the burning takes place in the fuel rich zones surrounding the droplets of injected fuel. This results in local hot spots and relatively high production of nitrogen oxides and other regulated pollutants.
On the other hand, Otto Cycle gasoline engines, such as automobile engines, burn fuel after it has been well mixed with air. This mode of burning fuel is called premixed combustion. In this mode, the fuel is pre-mixed with air to form a combustible mixture. The mixture is compressed in the compression stroke of the engine. The compression ratio is limited to a lower value to avoid premature ignition that results in hazardous “detonation” or “knock”. This lower compression ratio results in lower cycle efficiency. However, the premixing provides better control of the local combustion air to fuel ratio. This reduces local hot spots and lowers the production of nitrogen oxides and other regulated pollutants. The conventional gas engine operates in this combustion mode. Ignition of the fuel mixture is timed by a spark. Usually, the gas fuel is introduced at low pressure either into the intake manifold or directly into the engine cylinder before the compression stroke.
There are other combustion options that blend the advantages of conventional Diesel and Otto cycle engines. For instance, some dual fuel gas engines, called high pressure injection gas engines, have become known in the art. They also utilize a diffusion combustion mode, with an ignition source that is used to ignite the fuel.
More than 65 years ago it was recognized that a small amount of readily ignitable pilot fuel could be injected in diesel engines to improve combustion of “heavy” hydrocarbon fuels that are otherwise difficult to ignite. See British Patent No. 124,642. As used herein, the term “pilot fuel” means relatively light hydrocarbon fuel (e.g. methanol or even standard diesel fuel oil) characterized by being significantly easier to ignite than the primary fuel in the injection system.
Homogeneous charge compression ignition (HCCI) engines are a hybrid of gasoline and diesel engines in which HCCI engines offer high efficiency and very low emissions compared to diesel engines. However, HCCI engines rely on autoignition of their air-fuel (A/F) mixture and are difficult to control ignition thereof during compression. Ignition timing in HCCI engines is currently regulated by modulating the temperature or richness of the fuel mixture in the engine cylinder. Exhaust gas recirculation and modulation of the input charge aftercoolers are typical means of achieving this end.